The following meanings for the abbreviations used in this specification apply:    ABS Almost Blank Sub-frame    BTS Base Transceiver Station    C-RAN Cloud Radio Access Network    CRS Cell specific Reference Signal    CSI Channel State Information    DL Downlink    eICIC enhanced Intercell Interference Coordination    GBR Guaranteed Bit Rate    LTE Long Term Evolution    MIB Master Information Block    OPEX OPerational EXpenditures    PCI Physical Cell Identifier    PDCCH Physical Downlink Control CHannel    PDSCH Physical Downlink Shared Channel    PRB Physical Resource Block    PSS Primary Synchronization Signal    QoS Quality of Service    RLM Radio Link Monitoring    RRM Radio Resource Management    RSRP Received Signal Received Power    RSRQ Received Signal Received Quality    SIB System Information Block    SSS Secondary Synchronization Signal    TTI Transmission Time Interval    UE User Equipment    UL Uplink    3GPP 3rd Generation Partnership Project
Embodiments of the present invention relate to LTE and LTE-A radio access. In current LTE radio access networks there are two major trends for improving system capacity: namely, support of heterogeneous networks that are composed of macro, micro, pico and femto eNode Bs, and also centralized baseband processing where a large number of radio heads are connected to a central processing unit (also the terms C-RAN or baseband pooling/baseband hotelling refer to this kind of network deployments).
In the following, micro, pico and femto cells are referred to as small cells. Heterogeneous networks will deploy small radio cells/base stations in hot spot areas with high traffic demand like train stations, town centres, office areas etc in addition to the existing macro layer that provides the basic LTE coverage. The small cells might use the same or a different frequency layer. 3GPP has defined the so-called eICIC concept to enlarge the small cell coverage for those scenarios where both cells work on the same frequency layer. For this scheme a number of downlink sub-frames are not used by the macro base station and therefore the small cell base station will not be interfered in those sub-frames by the macro base station (some interference from reference symbols, synchronization symbols etc will remain and could be at least partly cancelled by the UE to enhance the performance). This allows the small cell base stations to serve in those sub-frames UEs that are located in the so-called cell range extension area that is just outside of the normal small cell coverage. This allows a better load balancing between the macro and small cells.
FIG. 1 illustrates the usage of the sub-frames in the macro and the small cells in the normal coverage and the small cell coverage. As shown, there are some subframes defined as ABS for the macro cell, and these subframes are preferably used for UE's in small cell range extension.
Another trend in future LTE network deployments concerns centralized baseband processing deployments where a number of different radio heads with different output power levels (serving macro/micro/pico or femto cells) are connected to a central baseband processing unit as shown in FIG. 2. In this example, the centralized baseband processing is carried out for a macro cell and also for small cells #1 to #3, which may be located in the geographical area of the macro cell.
Such schemes offer a number of advantages like                Centralized operation and maintenance saves OPEX        Baseband pooling gains        Simplified implementation of cooperative radio resource management schemes        
A basic problem of the above described eICIC scheme is that the macro cell will lose resources since DL transmissions are neither allowed on PDSCH nor on PDCCH. This leads to a loss of DL resources for the macro cell in proportion to the number of almost blank sub-frames. As a side effect there will be no dynamically scheduled uplink transmissions 4 sub-frames after a DL ABS since no PDCCH transmission were allowed during the ABS. Only semi-persistent UL scheduling allocations will be possible for those UL sub-frames.
Nevertheless, this scheme could still provide better system capacity and especially better cell edge throughputs since either those resources can be used by several small cells that are under the coverage of the considered macro cell or there are many users close to a small cell location in a so-called hot spot area. In both cases it is advantageous to reduce the capacity of the macro cell and boost the capacities of the small cells.
However, in real network deployments there will be a lot of different scenarios and the load of the macro and small cell layer will change dynamically due to mobility of the users and/or varying traffic demand.
Hence, the load has to be considered. In the following, some procedures regarding load collection and change of an ABS pattern according to the prior art are described.
3GPP has defined a number of procedures over the X2 interface that allow the exchange of load information as well as the negotiation of appropriate ABS patterns that will be employed by macro and small cells. Within the load information message two different information elements—the invoke information element and the ABS information element—have been defined to trigger (by small eNode B) and distribute ABS pattern information (by macro eNode B). Furthermore, within the resource status request/response messages the ABS status has been added to check the usage of the ABS in different radio cells which is a load measure for the ABS usage in the small cells.
Those procedures can be used to collect load information in the macro cell on the underlying small cells and decide on suitable ABS patterns and distribute the ABS information to the associated small cells. However, such layer 3 signalling requires some time and therefore ABS patterns can be changed in the range of a few minutes or so since the cell extension of the small cells needs to be adjusted also in response to the modified ABS pattern.
The adjustment of the cell range extension requires an estimation of how much spare capacity a certain neighbour cell has available. This can be checked via the composite available capacity information element that can be exchanged via the X2 interface as part of the resource status request/response/update procedures. Based on the collected load information it is possible to negotiate different values for the cell range extension via the mobility change procedure over the X2 interface. Finally these modified range extensions need to be converted to appropriate cell individual offset parameters that are then signalled to a subset or even all UEs in order to achieve the appropriate load balancing between the radio cells.
Thus, the described load adjustments by layer 3 signalling have the following drawbacks:                Cannot be too fast since it is handled by radio layer 3 protocols        Create a high signalling load if they are done too often        Cannot track fast load changes        Some definitions like the definition of composite available capacity are not so precise that a very good load balancing in a multi-vendor environment can be achieved.        Use of a larger range extension does not directly mean that the load changes in the same way since that depends very much on the location of the UEs around the considered small cell.        Cannot track the large load imbalances between different small cells        Cannot track the high load variations in small cells that are caused by the relatively low number of UEs per small cell (typically there are only a few UEs per small cell)        